The electroseismic method is a geophysical prospecting tool aimed at creating images of subsurface formations using conversions between electromagnetic and seismic energy. The electroseismic method is described in U.S. Pat. No. 5,877,995 (Thompson. et al.). The essence of the electroseismic method is that high levels of electrical energy are transmitted into the ground at or near the surface, and the electrical energy is converted to seismic energy by the interaction of underground fluids, including hydrocarbons, with the rock matrix. The seismic waves are detected at or near the surface by seismic receivers. To be effective, this method requires an input current source with the following characteristics:                The source should produce large current levels over extended time.        The source should have high electrical efficiency.        The source should contain little or no DC to avoid plating the electrode array.        The frequency content of the source should be adequate for the exploration needs.        The correlation of the source waveform with its reference should have sufficiently low side lobe levels.        
Little has been published to date on electroseismic waveforms because of the newness of the technique. However, in conventional seismic exploration, a seismic vibrator is sometimes used as an energy source to generate a controlled wavetrain (known as a sweep) which is injected into the earth. When the resulting recorded seismic data are correlated with the sweep wavetrain or other reference, the correlated record resembles a conventional seismic record such as that which results from an impulsive source.
When a source waveform is correlated with its associated reference, there will typically be a large peak at the onset time of the waveform surrounded by lower peaks at earlier and later times. These lower peaks are the correlation side lobes. Correlation side lobes are undesirable because they can mask smaller desired seismic returns.
It should be noted that the source waveform is just one piece of the electroseismic system. Other factors of importance include the power waveform synthesizer (that creates the waveform) as well as the input electrode array, the seismic receiver arrays and various field implementation issues.
As stated above, there is little current technology on electroseismic waveforms because of the newness of the technique. Some obvious approaches might include pulsing or pseudo-random square-wave sequences. Repeated pulses are inefficient (in energy/second) compared to continuous waveforms. Square-wave waveforms would be expensive to implement at the required high current levels and would also dissipate energy in unwanted high-frequency components.
What is needed is a source waveform that satisfies the requirements stated above. The present invention satisfies this need.